The attorney, Michael Weinstein, attended the hearing in Milford, Pennsylvania, to enter the plea on behalf of his client, while co-counsel William Ruzzo was with Frein at the jailhouse. Weinstein said his client appeared fine, and understood and answered questions asked by the judge.
Prosecutors are seeking the death penalty for Frein
, who is accused of shooting two state troopers
-- killing Cpl. Bryon Dickson and injuring Trooper Alex T. Douglass.
Frein faces a dozen charges, including terrorism and first-degree murder, in connection with the September ambush
outside the state police barracks in Blooming Grove, Pennsylvania.
Frein led a small army of law enforcement officers on a weeks-long manhunt in eastern Pennsylvania. The self-styled survivalist and military buff was captured on October 30
at an abandoned airport.
The manhunt lasted almost seven weeks, and involved as many as 1,000 officers at times. Authorities said the search cost more than $11 million in salaries, travel, equipment, utilities and other expenses.
Prosecutors had previously said they intended to seek the death penalty. They submitted the formal paperwork on Tuesday.
Frein's lawyer said the move was expected.
"There's a lot of work ahead, and there's just so much more to do," Weinstein said.
"We need to make sure we have everything analyzed correctly. Right now, it's all just in the early stages, so once we get the evidence laid out we can go from there," he said.
After his arrest, Frein allegedly told investigators he "wanted to make a change (in government) and that voting was insufficient to do so, because there was no one worth voting for," according to a court affidavit.
Frein called the killing of Dickson an "assassination," the court documents said. The suspect said the shootings were an attempt to bring about a change in government and to "wake up people, because it was all he could do."